1. Field of the Invention
The present invention relates to digital television (DTV) receivers, and more particularly to reliable synchronization detection circuits for use therein.
2. Description of the Related Art
Advances in technology now allow for the transmission of digital television (DTV) in the same bandwidth currently used by NTSC (analog) television transmissions. Digital transmission offers improved video and audio reception. The DTV standard for the United States was adopted on Dec. 24, 1996, and allows digital transmission of high quality video and audio signals, in particular high definition television (HDTV). Since the adoption of the digital television (DTV) standard, there has been an ongoing effort to improve the design of DTV receivers.
The primary challenge that faces designers in designing receivers so that they achieve good reception is the presence of multipath interference due to multiple signal paths in the channel. Such multipath interference affects the ability of the receiver to recover signal components such as the carrier and symbol clock. Therefore, designers add equalizers to receivers in order to cancel the effects of multipath interference and thereby improve signal reception. In a multipath environment, it is important to find quickly main path but sometimes it is impossible to determine properly the main path using receivers of the related art.
A DTV receiver includes a tuner, a demodulator, a filter, a sync detector and an equalizer. The data format of the DTV signals are shown in FIG. 1 and FIG. 2.
FIG. 1 is a diagram illustrating the timing structure and dimensions of a digital television (DTV) data field, comprised of a series of 313 segments, the first segment containing an equalizer training sequence, according to the related art. In ATSC DTV systems, frame of data is transmitted in a pair of “odd” and “even” data fields, a one data field being shown in FIG. 1. Each data field contains 313 segments, and each segment contains 832 symbols. The symbol rate for all digital VSB modes is 10.762 MHz and thus, the symbol period T is 92.9 nsec. Thus, in the DTV Frame Format: 1 frame=2 fields (odd field and even field); 1 field=313 segments=1 field sync segment+312 DATA segments.
The transmission (data) segments are compatible with the 188-byte MPEG-2 data packet standard, commonly used throughout the world (including the U.S. DTV standard). Twenty Reed-Solomon parity bytes for every data packet add redundancy for forward error correction (FEC) of up to 10 byte errors/packet. Since Reed-Solomon decoders correct byte errors, and bytes can have anywhere from 1 to 8 bit errors within them, a significant amount of error correction can be accomplished in the DTV receiver.
The first four symbols in each segment are “segment sync” symbols having the sequence [+5, −5, −5, +5]. The repetitive 4-symbol binary segment sync aids in symbol clock recovery and data segment delineation, independently of data. Due to the random nature of the data, the repetitive syncs can be easily extracted from the data through correlation methods, and can provide reliable synchronization down to S/N ratios of 0 dB.
Digital vestigial-sideband (VSB) DTV transmission systems use three supplementary signals for synchronization. A low-level pilot is employed for carrier acquisition, a segment sync (in each segment) for synchronizing the data clock in both frequency and phase, and a data field sync segment for data framing and equalizer training. The low-level pilot is created by adding a DC value to the baseband data, which has zero mean because all the VSB data levels (e.g. 2, 4, 8, or 16) are equiprobable. After modulation, the DC value causes an in-phase pilot to be added to the data spectrum for transmission. Carrier recovery in the receiver of a VSB DTV transmission system is performed using the low-level, inband pilot that is added to the random data signal. The pilot signal may be synchronously detected using a narrowband frequency-and-phase-locked loop (FPLL). A frequency and phase-locked loop (FPLL), combines both a frequency loop and a phase-locked loop into one circuit, and can be employed for both wideband frequency acquisition and narrowband phase tracking. When the FPLL is phase-locked, the detected pilot is constant. Thus, the low-level pilot aids carrier recovery independently of data.
A VSB data field pair (a DTV frame) comprises a first (odd) field (one data field) and a second (even) field (one data field), each of which includes a data field sync segment (the first segment in each data field), data, FEC, and segment syncs. The middle 63 PN sequence of alternate data field sync segments are inverted to identify (odd) Field Sync #1 and (Even) Field Sync #2. The remaining data in the other 312 segments comprises trellis coded 8-level VSB symbols. In a trellis-coded 8-VSB signal there are eight discrete data levels.
In the receiver in VSB DTV transmission system, sync and timing recovery is generally performed with a narrowband phase-locked loop (PLL), using the segment sync signal, independently of the data. Repetitive binary segment syncs, shown in FIGS. 1 & 2, provide the receiver with a means of extracting the clock signal from the otherwise randomized data signal. By using correlation techniques and a narrowband PLL tracking filter, the segment sync can be found and the symbol clock locked to it.
FIG. 2 is a timing diagram of the related art illustrating the timing structure and dimensions of the first segment of the data frame of FIG. 1 containing: a segment sync including a 4-symbol segment sync followed by a training sequence that includes a pseudo-noise sequence having a length of 511 symbols (PN511) followed by three pseudo-noise sequences each having a length of 63 symbols (PN63) followed by 128 symbols which are composed of various mode, reserved, and precode symbols. The binary “VSB Mode” (2/4/8/16) Level ID, shown in FIG. 2, indicates the VSB mode selected for transmission. Thus, in the Field Sync Segment: 4 symbols are the segment sync; 700 symbols (PN511, 3 PN63) are the equalizer training signals; 24 symbols are the VSB Mode difference; and 104 symbols are Reserved.
The DTV data field sync (training) segment (shown in FIG. 2), is one segment long (832 symbols) and repeats in each data field (repeating every 313 segments). The data efficiency (of the data fields) is reduced by only 0.32% ( 1/313) due to the insertion of data field sync (training) segments. The data field sync (training) segment aids in data frame synchronization, again independently of data and down to S/N ratios of 0 dB. The frame sync (training) segment can also be used as a known reference training signal for the receiver equalizer, and as a means of determining received signal conditions (such as S/N ratio) and for determining the main path in a multipath environment.
As shown in FIG. 2, the field sync segment (the first segment of each data field) comprises the four segment sync symbols discussed above followed by a pseudo-noise sequence having a length of 511 binary symbols (PN511) followed in turn by three pseudo-noise sequences each having a length of 63 binary symbols (PN63). Like the segment sync symbols, all four of the pseudo-noise sequences are composed of (binary) symbols from the set {+5, −5}. The center PN63 sequence is inverted in alternate (even) data fields. The pseudo-noise sequences are followed by 128 symbols, which are composed of various mode, reserved, and precode symbols.
Because the first 704 symbols of each field sync segment are known, these symbols, may be used as a training sequence for an adaptive equalizer. All of the three PN63 sequences can be used only when the particular field being transmitted is detected so that the polarity of the center sequence is known.
The 511-symbol PN sequence is used in long equalizers, providing accurate channel (linear) distortion reduction over a large time length. In order to facilitate a short equalizer implementation, three 63-symbol PN sequences are transmitted in the frame sync.
FIG. 3A is a diagram illustrating exemplary correlation values (A, B, C, D, E) of the real part signal (detected at a real part equalizer in a DTV receiver) in a multipath environment. In the multipath environment (see multiple paths indicated by A, B, C, D, E in FIG. 3A), it is important to find quickly main path (indicated in FIG. 3A by the largest correlation value, B). Multipath signals (e.g., A, C, D, E) in the broadcast channel may arrive many symbols after the main signal (B). Since the clock recovery, segment synchronization, and frame synchronization are done independently of each other, and prior to the equalizer, near-theoretical equalizer performance is possible. Also, the use of VSB modulation generally requires only one real (in-phase) equalizer, not two complex ones, and thus the receivers of the related art generally have only one “real” (in-phase) (I-phase) equalizer. But if only the real part signal is used in the synchronization, sometimes it may be impossible for the receivers of the related art to determine properly the main path (B).
FIG. 3B is a diagram comparing a detectable correlation value of the real part signal of main path (B in FIG. 3A) to background NOISE. The correlation value B of the real part signal of main path exceeds a predetermined noise-threshold level, NOISE_TH, and so the receiver of the related art using only a real part equalizer can determine properly the main path (B). If there is no phase offset, the correlation value of the real part signal can represent the whole signal, so according to the correlation value of the real part signal, synchronization is determined. The path having a maximum correlation value (e.g., B) is considered the main path.
The sync detector of a receiver detects the strength and position of multipath signals using the PN511. The sync detector receives the demodulator's output signal which is a real part signal (I), calculates the correlation of the PN511 sequence, to find a main path, and then outputs a locking control signal and sync signals. As previously noted, there are two types of PN sequences (PN511 and PN63) that are a kind of training sequence or training signal.
FIG. 3C is a diagram comparing an undetectable correlation value of the real part signal of the main path to background noise where there is a phase offset. The correlation value B′ of the real part signal of main path does not exceed the predetermined noise-threshold level, NOISE_TH, and so the receiver of the related art using only a real part equalizer cannot determine properly the main path (B′). If there is phase offset, the correlation value of the real part of the main path signal is so small that it may be difficult of impossible to synchronize. If the real part signal level is lower than the noise (threshold) level because of the noise or multipath or etc., the sync detector of the receiver of the related art cannot find the main path, so the equalizing speed becomes slow and the performance of the equalizer is lowered.
FIGS. 4A, 4B, 5A, and 5B are timing diagrams showing correlation values. FIGS. 4A and 5A each depict correlations of the real (I) part signal of exemplary main paths. The correlation (a) in FIG. 4A is the Correlation value of Real PN511 with a Zero phase-offset. The correlation (c) in FIG. 5A is the Correlation value of Real PN511 with a 90 degree (rotated) phase-offset.
FIGS. 4B and 5B each depict exemplary correlations of the Imaginary (Q) part signal of exemplary main paths. The correlation (b) in FIG. 4B is the Correlation value of Imaginary PN511 with a Zero phase-offset. The correlation (d) in FIG. 5B is the Correlation value of Imaginary PN511 with a 90 degree (rotated) phase-offset.
FIG. 6 depicts combined correlations (power) of the Imaginary (Q) part signal of one exemplary main path, in the cases of Zero phase-offset or a 90 degree (rotated) phase-offset. In the case of the Zero phase-offset example (a) and (b) (FIGS. 4A and 4B), Real PN511 Correlation value is high and the Imaginary PN511 Correlation value is low, and the total power (correlation) is indicated by (e). In the case of the 90 degree phase-offset example (c) and (d), (FIGS. 5A and 5B) Imaginary PN511 Correlation value is high and the Real PN511 Correlation value is low, but the total is indicated by (e).
Because correlation values of the Real PN511 and Imaginary PN511 vary according to the phase offset (rotation), it is not correct to use only the Real PN511 Correlation Value. But in all cases, the power of the PN511 (summation of the square values of the real part signal and the imaginary part signal) is constant. So if the total (I & Q) power of the PN511 is used, it is possible to find the main path without considering the phase offset.